An ultrasound imaging apparatus transmits an ultrasonic beam generated from an ultrasonic transducer in an ultrasonic probe to a patient, receives an ultrasonic reflective wave produced according to difference in sound impedance of a tissue of the patient with the ultrasonic transducer, and displays an ultrasonic image on a monitor. Since a 2-dimensional image can be observed easily in real time by easy operation, such as contacting the ultrasonic probe to a body surface, the ultrasound imaging method is widely used for functional diagnosis or morphological diagnosis of various internal organs of the patient.
As an example of a scan method of the ultrasonic imaging apparatus, there is an electronic scan method. In the electronic scan method, timing of transmission of the ultrasonic wave and timing of generation of received signals of ultrasonic transducers that are arranged at a tip of the ultrasonic probe are controlled by electronic switches and delay circuits, in order that an direction of the ultrasonic beam according to an arranging direction and a focus position are freely changed.
From the focus position where the ultrasonic beam is most focused, relatively stronger reflective wave of the ultrasonic beam can be obtained than that from other positions. In other words, a high resolution image is obtained near the focus position, and a low resolution image is obtained on other positions far from the focus position.
Predetermined amount of delay is given by each delay circuit connected to each ultrasonic transducer so that the focus position corresponds to a region to be observed in the patient.
However, it is difficult to recognize the focus position only from the image of the ultrasonic wave. In order to showing where the focus position located in the image, the ultrasound imaging apparatus, such as disclosed in Japanese Patent (Kokoku) No. 62-25375, is known. The ultrasound imaging apparatus includes a beam character recognition part that recognizes the focus position of the ultrasonic beam and a display part that displays the focus position recognized by the beam character recognition part. According to the ultrasound imaging apparatus, it is possible to recognize which portion is the highest or relatively higher resolution in the displayed ultrasonic image obtained by the transmission and reception of the ultrasonic wave.
By the way, in various cases, homogeneity of intensity of the ultrasonic wave in a transmission and reception direction of an ultrasonic beam or homogeneity of resolution is important. For example, in a technique called Contrast Echo method, a harmonic signal generated by destruction of small air bubbles injected into the patient according to a high power ultrasonic wave is measured. Unless the high power ultrasonic wave is uniform, destruction of the bubbles is not performed homogeneously, and a blood vessel to be observed cannot be recognized correctly.
In Color Flow Mapping (CFM) method, information of flow velocity, such as blood flow velocity, obtained by Doppler method, is superimposed on the ultrasonic image that shows a shape. Also in this method, the homogeneity of quality of image is required since comparison of the blood vessels to be observed is performed in a wide range.
Furthermore, when a biopsy needle is inserted into the patient, the ultrasound imaging apparatus is used for checking a location of the inserted needle. In this case, it is desirable that a wide range from a surface of the patient to a portion which the biopsy needle is injected to is displayed homogeneously in order that an insertion path can be easily recognized.
Thus, in various methods, homogeneity of intensity of the ultrasonic wave or whole homogeneity of resolution of image is more important than local high resolution of image. Therefore, when using the above mentioned conventional methods, it is generally known that defocusing ultrasonic beam is used or the ultrasonic beam is repeatedly transmitted or received several times during changing the focus position, called as multi focus method, to improve the homogeneity of the image by homogenizing the resolution of the image.
Also in this case, the technology disclosed in Japanese Patent (Kokoku) No. 62-25375 is useful. By referring displayed beam characteristics and adjusting the focus position, a desired quality of image can be obtained.
By the way, in a 2-dimensional array probe proposed in recent years, ultrasonic transducers arranged in 2-directions, and delay control can be performed in the 2-directions. For example, when the homogeneity of the image should be important as mentioned above, the focus position is controlled in the 2-directions independently, two focus positions are located in different locations between region of interest. Thereby, the region of interest is displayed homogeneously.
A method where the beam focus positions can be located in different locations in 2-directions is useful, especially when the homogeneity of the image is improved by beam defocusing method or the multi-focus method, since the both methods have problems, such as deterioration of image or frame rate.
However, in the conventional technology as disclosed in Japanese Patent (Kokoku) No. 62-25375, an ultrasonic scan is performed only one direction along an arrangement of ultrasonic transducers, and a single focus position is displayed in the direction.
In other words, Japanese Patent (Kokoku) No. 62-25375 does not disclose the focus positions of the 2-dimensional array probe are controlled and displayed in 2-dimension.
Even if the 2-dimensional array probe is adopted to the conventional technique as disclosed in Japanese Patent (Kokoku) No. 62-25375, it is difficult to recognize and set the focus positions in 2-dimension is consideration for relationship of the focus positions.